I have a NIB .45acp AMT Back-Up.
They are a quirky gun. Some shoot just fine, and others.....well, not so much. I think a lot of it has to do with limp-wristing and lack of a firm grip. I've heard that they don't like hollow-point rounds.
Anyways, I've fired a friend's, chambered in 9mm. It feeds, functions and shoots fine. At one time, I was gonna shoot mine, but decided not to, and keep it as unfired, in case I decide to sell it in the future.
They were available in quite a few calibers as I recall. Anyways, since the one you show is complete, it may be worth a pretty penny. A quick check on GunBroker will pretty much show you what they are selling for.
I do think that the AMT Back-Up series will increase in value as the years go by.
Another little tid-bit. Tell your father's friend to buy some extra factory mags while they are still available.